RFID systems comprise an RFID writing/reading device (reader) and electronic tags. The latter can operate passively, i.e. without a battery, and are therefore reliant on the permanent presence of a carrier signal transmitted by the RFID writing/reading device, also called a power carrier. The carrier signal simultaneously serves as a radio-frequency oscillator for the tag. Although semi-passive tags have a battery, they likewise need a permanently transmitted carrier signal instead of a radio-frequency oscillator for modulation. In this context, RFID systems with ranges of several meters use UHF frequencies or microwave frequencies. The RFID writing/reading device itself comprises a baseband part and a radio-frequency part (RF part) having a transmitter and a receiver.
To achieve reading distances in the region of several meters for passive electronic tags and in the region of several tens of meters for semi-passive electronic tags, a transmitted signal provided by a transmitter TX needs to be produced and emitted at a power of approximately 1 watt (30 dBm). In this case, a significant part of this comparatively powerful transmitted signal is spuriously injected directly into the receiver RX, see FIG. 1, the coupling paths 28, 29 and the reflection at a reflector 27. On the other hand, the receivers in the RFID writing/reading devices must detect the low levels of the response signal which is reflected unamplified from the tags only after modulation of the carrier signal with response data. Such a resultant, requisite wide dynamic range in the receiver in the RFID writing/reading device represents an enormous demand on any bidirectionally operating system. In this case, the RFID writing/reading device can either be operated using a single antenna, with the transmitter and the receiver being decoupled by a circulator, or two directly adjacent antennas are used. In both cases, the isolation between the transmitter and the receiver is known to be low; typically, such isolation is merely around 20 dB.
The transmitted signal injected directly from the transmitter into the receiver is therefore not only very powerful but also unwanted, since it results in intermodulation with the response signal from an electronic tag in the radio-frequency stages of the receiver and hence reduces the sensitivity of the receiver. In other words, the isolation from the transmitter TX to the receiver RX for an RFID writing/reading device is much too low for long ranges in practice in the UHF and microwave band. The isolation is determined by the design of the RFID writing/reading device and particularly by the technology of the circulator at the antenna output or by the arrangement of the transmission and reception antennas used. The use of a circulator can subsequently be handled in the same way as the use of separate antennas. The latter is discussed here as representative of both uses.
A numerical example demonstrates the problem explained above as follows: if the transmitted power is 30 dBm with isolation of 20 dB, this results in a spurious signal of +10 dBm at the receiver input. The useful signal emitted by a passive electronic tag is only just −70 dBm, however, in the UHF band at a distance of approximately 4 m. The high level of +10 dBm for the transmitted signal injected directly into the receiver thus overtaxes the input amplifier of any RFID writing/reading device, which is typically a low-noise small-signal amplifier.
Intermodulation frequencies arise between the injected transmitted signal and the received signal from electronic tags or the signals from other RFID writing/reading devices which are active at the same time. If the received signal is digitized for further evaluation, a dynamic range of 80 dB additionally gives rise to a problem with the available resolution in the case of the 14 to 16 bit analog/digital converters which are usual for this. If the desire is merely to solve the problem by improving the components, the demands on the RF components and the A/D converter become very high and unappealing, particularly as far as linearity and power consumption are concerned. Suppression of or electronic compensation for the injected transmitted signal is thus necessary in order to achieve long ranges.
A known reception architecture therefore provides a so-called direct conversion stage (also called DCS below) in order to move from the radio frequency RF to a baseband. If the data on the electronic tag are modulated and reflected without direct current, the dynamic range for the A/D converter can be moderated by filtering away the DC voltage component DC after down-mixing in the receiver's DCS. In the RF input part of a receiver, however, nothing changes in the intermodulation situation, and electronic compensation is still required. Since removing the DC voltage component eliminates the contribution of the injected transmitted signal in the baseband, said contribution must be detected and processed in an additional circuit. In practice, a maximum received signal with a level of −10 dBm, for example, would be desirable. On the basis of the above calculation, an isolation of at least 20 dB is therefore additionally necessary.
US 2004/0106381 and U.S. Pat. No. 6,229,992 B1 propose measuring the injected signal in the receiver, comprising a reception antenna 11, an addition stage 8, an RX converter 4 and an A/D converter 2 in FIG. 2a, and adding a compensation signal, which is derived directly from the transmitted signal in the RF part, in the reception path shown.
As is known, a reading device 19 comprises a software-defined baseband part 1 (also called SDR below) and an RF part 18, as shown in FIGS. 2a, 2b. In such SDR-based transmission/reception installations, the complex-value signals are conditioned or processed purely by computational means in a signal processor 6 to the extent that they now need merely be shifted by means of linear converters (up converter or down converter) to, or from, the radio-frequency band (RF band). A TX converter 5 in the transmitter is fed a complex baseband signal (inphase and quadrature signal, also called I/Q signal below) which is output by the signal processor 6 via a dual digital/analog (D/A) converter 3. The output signal is forwarded to a transmission antenna 12 via a directional coupler 7. From a reception antenna 11, the received signals are converted into a complex baseband signal (inphase and quadrature signal) by means of an RX converter 4 and are forwarded to a dual A/D converter 2 and accepted by the signal processor 6. In the conventional solution, the transmitted signal is obtained from the output RF signal from the directional coupler 7 and is weighted in the vector modulator 10 with the correction values for phase and amplitude from the DSP 6 using a slow D/A converter 9 and is supplied to the received signal in an addition stage 8 for the purpose of compensation.
In a first step of the reading operation, the received signal is usually digitized and analyzed in a brief Listen Before Talk Phase (LBT phase) of an interrogation cycle over a time interval T0 when the transmitter in a first RFID writing/reading device is turned off. This signal contains the interrogation signals from further RFID writing/reading devices, and it is possible to decide whether or not the transmitter in the first RFID writing/reading device can be turned on. If the first RFID writing/reading device is switched to transmission after the LBT phase, its own receiver must first of all compensate for the injected TX signal in order to achieve a high level of sensitivity.
The compensation signal is obtained from the transmitted signal by adjusting the gain and phase (gain/phase adjuster). This technique is known as adaptive filtering. In this context, the amplitude and phase are adjusted using the so-called vector modulator 10 totally in the RF band. The components for outputting the transmitted signal, for addition in the reception path, and also the vector modulator 10 are RF components, however, which themselves have inaccuracies and features which are not ideal. Immediately effective (instantaneous) reduction of the coupling therefore appears possible only with difficulty and in practice is able to be implemented only iteratively in cycles: measurement/compensation/measurement, etc.
WO 2006037241 uses an architecture with digital signal processing according to FIG. 2b; it is thus possible to compensate for the spurious signal in the receiver by generating a correction signal using a digital signal processor 6, followed by a D/A converter 20 and a linear modulator 14. The linear modulator 14 must operate with very little noise since its noise contribution is not correlated with the transmitted signal to be compensated. The noise of the transmitted signal thus cannot be reduced further with this device and so the carrier signal (cf. 21 in FIG. 3) alone must also be accordingly generated with little noise.
US 2006033607 describes a plurality of devices which are based on directional couplers, are intended to suppress the transmitted signal and comprise at least two directional couplers and one vector modulator for solving the problem. One directional coupler is essentially used to output the transmitted signal and the second is used to add the received signal to the compensation signal from the vector modulator. The principle of compensation using a vector modulator has already been disclosed, in principle, in GB 1510625.
U.S. Pat. No. 5,691,978 proposes combining antenna isolation, analog RF suppression and a digital “echo canceller” in the baseband in order to achieve a high level of isolation. The outlay for this is considerable and makes no sense in economic terms for an RFID writing/reading device.
A common feature of all of these known methods, except for that in WO 2006037241, is that, if a receiver designed on a DCS basis is used, it will be necessary to evaluate a DC voltage signal (DC signal) in order to obtain information about the amplitude and phase of the transmitted signal being injected. However, this DC signal is in turn itself subject to errors as a result of coupling effects on the RF mixers in the RX converter. The same applies to methods which detect crosstalk using a conventional envelope detector. Additional problems arise when other RFID writing/reading devices unintentionally transmit on the same frequency channel at the same time, since their transmission frequencies possibly differ only slightly and thus corrupt each measured value.
For the purpose of measuring suppression, WO 2006037241 proposes generating an auxiliary signal 22 in order to be able to use AC coupling 16 in the receiver 4. This measure prevents overdriving of the low-frequency receiver part as far as the A/D converter and reduces the demand on the dynamic range of the A/D converter. If a transmitted signal from a further interfering RFID writing/reading device is present, it can be argued that calibration cannot be carried out at all under certain circumstances. This problem is solved either by means of the rule for using LBT or by means of synchronization of the entire RFID writing/reading device network.